The present invention relates to the field of acoustic communications. More particularly, the invention relates to the measurement of one or more parameters of the acoustic field for information recovery.
Acoustic waves have been used for target localization and SONAR applications for many years, especially in underwater applications. The steady growth of ocean exploration activity in recent years has resulted in a rising need to convey data through underwater channels. Numerous applications of acoustic communications pose an increasing demand on high-speed underwater wireless telemetry and data communication systems. These systems often require a combination of sensors, autonomous underwater vehicles, moored instruments, and/or surface ships to communicate with each other. Examples of such applications include: real-time remote monitoring of underwater tools, construction, and/or environmental factors in the offshore oil industry, continuous observation of ocean phenomena over geographically large areas, observation of fisheries, as well as many naval and security applications, including, but not limited to harbor monitoring systems and tactical surveillance operations.
Underwater communication systems generally use acoustic waves to convey information. In the underwater environment, electromagnetic waves do not propagate, as they attenuate rapidly.
In general, underwater acoustic channels are bandwidth-constrained. For distances from 10 km up to 100 km (long range), the available bandwidth is about a few kHz, whereas in a 1-10 km medium range setup, the available bandwidth is almost a few 10 kHz. Communications over short ranges, smaller than around 100 m, may have an available bandwidth exceeding 100 kHz. Underwater communication may be complicated by the harsh multipath conditions, and/or channel-alone time variations due to water surface fluctuations, internal waves, and/or turbulence. The multipath conditions may result in delay spreads up to several hundreds of symbols for high data rates. Further, channel-alone time variations may result in Doppler spreads up to several 10 Hz. After the first generation of underwater (analog) modems, second generation (digital) modems used non-coherent techniques such as frequency shift keying (FSK) and differentially coherent schemes like differential phase shift keying (DPSK). Due to the need for higher spectral efficiencies over typical channels of interest, coherent systems with phase shift keying (PSK) and quadrature amplitude modulation (QAM) were also developed.
Data rates available from existing systems are much lower than the data rates required for the real-time transmission of data, such as video and telemetry signals, over medium and long distances. For example, a typical commercially available modem provides only up to 2400 b/s at a 2 km depth and 3 km range setup.
Traditionally, underwater acoustic transmission has been limited to the scalar component of the acoustic field, i.e., the pressure. Existing multichannel underwater receivers are, generally, composed of spatially separated pressure-only sensors resulting in large size arrays. Array size is a limitation in modem applications, especially for small autonomous underwater vehicles. For example, the medium frequency (MF) 3 kHz receive array of a modem designed for a 21-inch diameter autonomous underwater vehicle includes four hydrophones and is 1.5 m long. For smaller size autonomous underwater vehicles, the necessary modem array may prove unwieldy.
In the past few decades, a large volume of research has been conducted on theory, performance evaluation, and design of acoustic vector sensors. These acoustic vector sensors have been used for the detection of acoustic signals, for example, underwater target localization and SONAR applications. For example, vector sensors have been studied for use in applications including accurate azimuth and elevation estimation of a source, avoidance of the left-right ambiguity of linear towed arrays of scalar sensors, and acoustic noise reduction due to a highly directive beam pattern.
The presently disclosed novel system and method include all of the same advantages present in traditional techniques but eliminate associated disadvantages.